A Note on FLAC -Compressing 24-bit

One note which I had commented about before my blog began, was that if authors decide to capture sound at 96k samples /second, the resulting sound should compress well using FLAC.

But now that I have experimented with ‘QTractor‘ and an external sound card, I have realized that we will probably also be capturing that sound in 24-bit sample-format, instead of 16-bit. And the sad fact is, that FLAC will not compress the 24-bit format as well, as it did 16-bit.

The reason seems clear. Using ‘Linear Predictive Coding’ means that FLAC will be able to predict the next sample in a set of so-many, to maybe 8 bits of precision, except that the next sample will always deviate from this prediction by a small residual. So 8-bit sound should compress brilliantly.

But then with 16-bit, the accuracy of the encoding stays the same. So again, the ‘LPC’ is really only 8-bits accurate at best, meaning that we get a larger residual. The size of that residual is what makes up most of a FLAC File.

Well at 24-bit, again, the LPC will only predict the next sample, accurately to within 8 bits. And so the residual is likely to be twice as large, as it was with 16-bit, completing 24-bit accuracy this time. We are not left with much compression then.

When I recorded my 14-second sound session the other day, I selected FLAC as my capture file format. I had a noisy air-conditioner running in the background. Additionally, the compression level defaults to Fastest, because the file needs to be written in real-time, and not chewed on.

At 96 kHz, 24-bit stereo, raw audio will take up about 4.6 mbps. At 44.1 kHz, 16-bit stereo, raw audio takes up about 1.4 mbps.

Well I was capturing to a stereo FLAC File, but was only using one channel out of the two. So the FLAC File that resulted, had a bit-rate of 2.3 mbps. This means that FLAC recognized the silent track and used ‘Run-Length Encoding’ on it, but that was about all this CODEC could do for me.

Now, we do have a command-line tool which will-re-compress that file:


$ flac -8 infile.flac -o outfile.flac
$ flac -8 infile.flac --channels=1 -o outfile.flac
$ flac -8 infile.flac --channels=1 --blocksize=8192 -o outfile.flac

The -8 means to use maximum compression.

For me, the bit-rate went down to 2.2 mbps either way.

It beats using a raw format, because using the latter would have meant, nothing would have detected my silent stereo channel, and the file would have been twice as large.

Dirk

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